Thermographic print head and method of making same

ABSTRACT

A thermographic print head for encoding labels on thermographic paper is made by a method of depositing, on an electrically insulating substrate, first and third layers of electrically conductive material separated by a second layer of insulating material. The first layer comprises a plurality of separated terminal leads, and the third layer comprises a plurality of separated concentric ring conductors, each of which is connected to selected terminals of the first layer through vias in the separating second layer of electrically insulating material.

United States Paten [191 Vossen et al.

{54] THERMOGRAPHIC PRINT HEAD AND METHOD OF MAKING SAME [75] Inventors: John Louis Vossen, Somerville; John Joseph ONeill, Belle Meade, both of NJ.

[73] Assignee: RCA Corporation, New York, N.Y.

[111 3,736,406 51 May 29, 1973 3,578,946 5/1971 Colello ..2l9/2l6 3,495,070 2/1970 Zissen ..2l9/2l6 Primary Examiner-C. L. Albritton Attorney-Glenn H. Bruestle, H. Christoffersen and R. Williams [57] ABSTRACT A thermographic print head for encoding labels on thermographic paper is made by a method of depositing, on an electrically insulating substrate, first and third layers of electrically conductive material separated by a second layer of insulating material. The first layer comprises a plurality of separated terminal leads, and the third layer comprises a plurality of separated concentric ring conductors, each of which is connected to selected terminals of the first layer through vias in the separating second layer of electrically insulating material.

9 Claims, 12 Drawing Figures llllllllllll "lllllllllllll lllllll Patented May 29, 1973 3,736,406

3 Sheets-Sheet l Patented May 29, 1973 I5 Sheets-Sheet 3 Fig. 12.

BACKGROUND OF THE INVENTION DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown an embodi- This invention relates to a thermographic print head merit of the novel grap print h ad capaand a method of making same. The novel thermographic print head is particularly useful for encoding labels to be applied to packages, as in a supermarket,

for identifying the commodity, the manufacturer, the 1 quantity, and/or the proce of the package.

It has been proposed to provide packages with labels having a code that comprises a plurality of concentric rings which could be read with a laser beam, as, for example, in U. S. Pat. No. 3,622,758, issued on Nov. 23, 1971 to Joseph F. Schanne for Article Labeling and Identification System. A practical system of marking or encoding labels in a supermarket is one wherein the labels may be printed at a rate of about one per second with the same printing head. By using 40 code rings and about 3 or four flag rings, a 10 digit marking code can be utilized. For practical purposes, however, each printed ring width should be about 0.01 1 inch i 0.001 inch, and it should be uniform and reproducible. Several label printing systems have been investigated and found wanting for one or more of the reasons of high cost, slow speed, difficult changeability of printing patterns, and mechanical complexity. Some of the systems that have been tried included the use of preprinted labels, electrographic and electrophotographic techniques, and solenoid-operated ball point pens; but these prior art systems exhibited one or more of the aforementioned disadvantages.

SUMMARY OF THE INVENTION A novel thermographic print head comprises a multilayered structure on an electrically insulating substrate. A first layer of electrically conductive material comprises a plurality of separated terminal leads. A second layer of electrically insulating material is disposed over the first layer, and a third layer of electrically conductive material comprises a plurality of separated electrical conductors that contact selected terminals of the first layer through vias formed in the insulating second layer. In a preferred embodiment of the novel thermographic print head, the first and third layers ofelectrically conductive material comprise molybdenum, and the ratio of the sheet resistivity of the third layer to the first layer is between 2.7 and 10.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a planar view of an embodiment of the novel thermographic print head;

FIG. 2 is a planar view of an upper (third) metallic layer comprising a plurality of concentric rings of the thermographic print head shown in FIG. 1;

FIG. 3 is a planar view ofa lower (first) metallic layer comprising a plurality of separated terminal leads of the thermographic print head shown in FIG. 1;

FIGS. 4-11 are enlarged fragmentary portions, in cross section, of the thermographic print head in different stages in the novel method of making it; and

FIG. 12 is a schematic drawing of the thermographic print head with apparatus for printing a label on thermographic paper.

ble, when suitably energized, of printing labels on commercially available thermographic paper at a rate of about one label per second. The print head 10 comprises a multilayer structure on a substrate 12 of electrically insulating material, such as a disc of Pyrex glass. A first layer 13 (FIG. 3) on the upper major surface 14 of the substrate 12 comprises a plurality of separated, relatively narrow, elongated, terminal leads 16 and a relatively much larger common terminal lead 18 of electrically conductive material, as shown in FIG. 3.

The elongated terminal leads 16 extend in radial directions from a point in common and have terminal connecting portions 17 that are parallel to each other. The common terminal lead 18 has a semicircular area with a terminal connecting portion 20 extending therefrom.

A third layer 21 (FIG. 2) of electrically conductive material, comprising a plurality of closely spaced, separated conductors in the form of a plurality of concentric rings 22 around a circular dot 24, is disposed over the first layer 13 of the terminal leads 16 and 18 but is separated therefrom by a thin second layer 28 (FIG. 10) of insulating-material, as will be described in detail hereinafter. Each of the rings 22 is connected to the common terminal lead 18 and to a separate one of the terminal leads 16 through vias 30 (FIG. 8) formed in the separating insulating layer 28. The construction of the print head 10 will be understood better in considering the method of making it, as will be discussed with the aid of FIGS. 4-11.

In constructing the print head 10, several features must be considered. The print head 10 is essentially a thin film heater wherein each ring 22 has two terminal leads (l6 and 18) contacting it at points located apart. Because the rings 22 are concentric, the print head 10 is a multilevel metallization structure comprising the two (lower and upper) metallization layers 13 and 21 separated by the insulating layer 28, the latter layer having vias 30 (FIG. 8) etched therethrough for connecting the first and third metallization layers 13 and 21 at desired contact points.

The heater ring pattern should have a resistivity high enough so that moderate current densities can be used to heat them, but not so high as to require very high voltages. The insulating second layer 28 and the substrate 12 should have a high enough thermal conductivity so that the rings 22 can be cooled rapidly between printings, but not so high as to require very high voltages. Also, the insulating layer 28 and the substrate 12 should have a high enough thermal conductivity so that the rings 22 can be cooled rapidly between label printings, but not so high as to draw excessive heat from the rings 22 during the printing cycle. The terminal leads 16 and 18 should have a resistivity lower than that of the rings 22, but should not have such a low resistivity as to act as heat sinks during the printing cycle. The insulating layer 28 should be capable of withstanding several times the highest voltage used on the structure, and, the entire structure should be very rugged, scratch resistant, and not show significant parameter shifts with use.

Referring now to FIG. 4, there is shown a portion of the substrate 12 comprising Corning Code 7740 Pyrex glass. The substrate 12 has an excellant chemical resistance, and a moderate thermal expansion (3.3 X l /C), a high resistance to thermal shock, and can be processed at temperatures up to 490C without softening. The thermal conductivity of the substrate 12 is about 0.01 watts/cmK. A practical size for the substrate 12 is a circular disc of about 3 inches in diameter and one-eighth inch in thickness.

The first layer 13 of electrically conductive material is disposed on the upper surface 14 of the substrate 12, as shown in FIG. 5. The first layer 13 is a composite layer that comprises a relatively thin lower layer of a metal, such as chromium, titanium, hafnium, tantalum, or alloys thereof, for example, and a relatively much thicker upper layer of molybdenum. The relatively thin layer of the composite first layer 13 is first disposed over the surface 14 of the substrate 12, as by vapor deposition or by sputtering techniques, known in the art. This prevents the relatively thicker, upper layer of molybdenum from recrystallizing because it is necessary to deposit the molybdenum layer on a metallic base. Chromium is preferable to the other metals mentioned supra for the thinner, lower layer because the etchant for molybdenum and chromium can be the same. The thicker upper layer of molybdenum is disposed on the chromium layer by sputtering with an rf induced bias (-150V), as taught, for example, in U. S. Pat. No. 3,640,812, issued to J. L. Vossen, Jr. on Feb. 8, 1972 for Method of Making Electrical Contacts on the Surface of a Semiconductor Device, and incorporated herein by reference. The chromium layer is between 500 and 1,000A thick, and the molybdenum layer is about l0,000A thick.

The terminal leads 16 and 18, shown in FIG. 3, are formed by photolithographic techniques, well known in the art. For example, the first (composite) layer 13 is covered with a suitable photoresist, exposed with an image from a photomask to produce the desired terminal lead patterns, and then etched with a suitable etchant to provide the terminal leads 16 and 18, portions of which are shown in FIG. 6.

The second layer 28 of electrically insulating material, is disposed over the terminal leads l6 and 18 of the first layer 13, as shown in FIG. 7. The second layer 28 has a dielectric strength that is at least 3 or 4 times that required for the maximum working voltage (about 100 volts) of the print head 10, and is extremely rugged and readily etchable. The layer 28 possesses substantially the same thermal conductivity as that of the substrate The second layer 28 is a composite of a relatively thin lower layer of about 5,000A of rf sputtered Pyrex" glass and a relatively much thicker upper layer of a chemically vapor deposited borosilicate glass having a thickness of about 45,000A. The composite second layer 28 ofPyrex" and borosilicate glasses reduces the possibility of the second layer 28 cracking. The rf sputtered Pyrex glass over the terminal leads 16 and 18 provides a uniform surface upon which the chemically vapor deposited borosilicate glass can nucleate. The preferred composition of the borosilicate glass layer of the second composite layer 28 consists of between 15-20 mol% B 0 and 85-80 mol% SiO The second layer 28 is covered with a photoresist, exposed through a suitable photomask, and etched by photolithographic means well known in the art to provide a plurality of vias 30 therein that extend to, and expose, the terminals 16 and 18, as shown in FIG. 8. A portion of the second layer 28 is also etched away to expose the parallel terminal connecting portions 17 of the terminal leads l6 and the terminal connecting portion 20 of the common terminal lead 18 so that electrical connections can be made thereto for electrically energizing the print head 10.

The third layer 21 of electrically conductive material, such as molybdenum, is sputtered with an rf induced bias (-200V), in accordance with the aforementioned U. S. Pat. No. 3,640,812, onto the second layer 28, covering all surfaces of the second layer 28 and contacting the terminal leads 16 and 18 of the first layer 13 through the vias 30, as shown in FIG. 9. The third layer 21 is deposited to a thickness of about 3,500A. The third layer 21 is sputter deposited onto the second layer 28 in the manner described in the aforementioned patent because such a deposition method enables the sputtered metal to extend into the vias 30 and to make good electrical contact with the terminals 16 and 18.

A heater ring pattern of a plurality of concentric heater rings 22, such as shown in FIG. 2 of the drawing, is formed in the third layer 21. To this end, the third layer 21 is covered with a suitable photoresist, exposed through a photomask for forming the desired heater ring pattern, and etched by photolithographic means well known in the art to form the concentric pattern of heater rings 22, a fragment of which is shown in cross section in FIG. 10. Each ring 22 is connected to both the common terminal lead 18 and to a separate one of the smaller terminal leads 16. Also, the two connections to each ring 22 are spaced 180 apart with respect to the circular dot 24.

Molybdenum is the preferred metal for the rings 22. More conductive metals either do not stick well to glass or are subject to electromigration. In addition, oxides stick well to molybdenum. The only possible disadvantage of using molybdenum in the print head 10 involves oxidation. Molybdenum oxidizes in air completely at 500C, and the oxide completely vaporizes at 650C. However, since the oxidation temperature is approximately twice the operating temperature (about 250C) of the print head 10, the aforementioned disadvantage is felt to be inconsequential.

The properties of the heater rings 22 largely dictate the properties of the other materials used in the print head 10. The heater rings 22 are 0.01 inch wide and are separated by 0.001 inch on the print head 10. The optimum range of sheet resistivities for the heater rings 22 has been determined as between 0.5 and 0.75 ohm/- square. This range limits the highest operating voltage (voltage on the outer ring 22) to about between and volts. By depositing the third layer 21 by sputtering with an rf induced substrate bias (-200V), as described in the aforementioned U. S. Pat. No. 3,640,812, the resistivity can be controlled. Under these sputtering conditions, the resistivity of the molybdenum is approximately 24 X 10 ohm cm. Therefore, the range of thickness of the rings 22 as required for the desired sheet resistivity of the layer 21 is between 2800A and 4600A. This is a relatively broad range and easy to con- X A/cm This value is considerably below that at which electromigration becomes a problem with molybdenum. The stresses in sputter-deposited molybdenum are relatively low (approximately 10 dynes/cm and are compressive under the aforementioned method of deposition. This is not always true for other deposition methods.

Aside from resistivity and stress control, any poor connection between the first and third layers 13 and 21, through the vias 30, can result in hot spots and eventual opens in the print head 10. This is avoided by making the first layer 13 approximately 10,000A thick. Also, the rings 22 must cross about 2024 steps over this structure without cracking, notching, or thinning. In addition, there are about 88 vias 30 in the print head 10, and each via height is between 4.5 and 5.0 pm. The walls of these vias 30 must be uniformly coated. To insure perfect edge coverage, a delicate balance exists between the sputtered deposition rate and the rf bias, as described in the aforementioned patent and also in the Journal of Vacuum Science and Technology, Vol. 8, page S12 (1971). The rf bias voltage is chosen to maximize small angle re-sputtering to insure side-wall coverage, and the deposition rate is held down to about 37A/minute.

The heater rings 22 may be left bare, but since they are very closely spaced, dust particles can gather between them and cause them to are over and burn. To prevent this, a fourth layer 34 in the form of a cap layer of rf sputtered Pyrex is deposited through a mechanical mask over the heater rings 22. The fourth layer 34 is about 6,000A in thickness.

It was determined emperically that the ratio of the heater rings sheet resistivity to the terminal leads sheet resistivity must lie between 2.7 and 10. If this ratio islarger (i.e., the terminal leads 16 and 18 are more conductive), the terminal leads l6 and 18 heat sink the heater rings 22 during printing. This, then, necessitates overdriving the rings 22, to carry out the printing function, producing several detrimental side effects as follows:

1. Higher power levels are required to print.

2. Higher voltage break-down strength is required of the insulating layer 28.

3. The segments of the rings 22 that are not heat sunk are overheated and print broader lines on thermographic paper.

4. The overheating extracts resinous material from the thermographic paper on which the labels are printed, and the extracted material polymerizes on the print head 10, eventually causing arcing between the rings 22 and ruining the print head 10.

On the other hand, if the terminal leads 16 and 18 are not conductive enough, they get hot enough to print. The safe range for the sheet resistivity of the terminal leads l6 and 18 is between 0.075 and 0.1 85 ohms/per square.

Referring now to FIG. 12 of the drawing, the thermo graphic print head 10 is shown schematically with apparatus for printing labels on a web of thermographic paper 36. The web of thermographic paper 36 is unwound from a feed roll 38, mounted for rotation about a fixed spindle-40, and wound up on a take-up roll 42, mounted on a power-driven spindle 44 for rotation therewith. The print head 10 is mounted parallel to, and slightly spaced from, one major surface of the thermographic paper 36.

A resilient mat 46, having a cross-sectional diameter substantially equal to the overall diameter of the ring pattern of the print head 10,-is disposed boh adjacent to the ring pattern of the print head 10 and adjacent the opposite major surface of the thermographic paper 36, as shown in FIG. 12. The mat 46 is adapted to be moved reciprocally, perpendicularly to the print head 10, in the directions indicated by the doubleheaded arrow 48, to periodically press the thermographic paper 36 against the print head 10 for printinglabels on the thermographic paper 36.

Electrical leads, such as a lead 50 connected to the terminal connecting portion 20 of the common terminal lead 18 and a plurality of leads 52 (only one being shown in FIG. 12), each connected to a different one of the terminal connecting portions 17 of the terminal leads 16, are connected to a suitable source of electrical energy (about volts or less) through suitable energizing and switching means (not shown). Thus, the rings 22 can be selectively energized in accordance with a predetermined code, to heat selected rings 22. The thermographic paper 36 is thermally affected by the selected heated rings 22 and provides a coded, printed label thereon. The mat 46 presses the thermographic paper 36 against the print head 10 every time the print head 10 is energized for printing acoded label.

Under the conditions described supra, the printing temperature is approximately 250C, and the heater rings 22 are cycled over a 225C range once per second in operation. This implies that all of the parameters in the manufacture of the print head 10 must be well controlled, since a single defect in the involved method will render the print head 10 inoperative.

We claim:

1. A thermographic print head comprising:

an electrically insulating substrate,

a first layer of electrically conductive material comprising a plurality of separated terminal leads disposed on a surface of said substrate,

a second layer of electrically insulating material disposed over portions of said separated terminal leads and portions of said surface of said substrate between said terminal leads,

a plurality of vias formed in said second layer, each of said vias extending to said first layer, and

a third layer of electrically conductive material comprising a plurality of separated conductors disposed over said second layer, each of said conductors extending to, and contacting, two of said terminal leads through said vias.

2. A thermographic print head as described in claim 1, wherein a fourth layer of electrically insulating material is disposed over said conductors.

3. A thermographic print head as described in claim 2, wherein each of said second and fourth layers comprises sputtered glass having a melting point in excess of 490C and a thermal expansion of about 3.3 X 10 /C, and

said second layer is a composite of a bottom layer of said sputtered glass and a top layer of chemically vapor-deposited borosilicate glass.

4. A thermographic print head as described in claim 1, wherein 7 8 said plurality of conductors comprises a plurality of posed over said rings.

concentric rings, and 6. A thermographic print head as described in claim said plurality of terminal leads comprises a relatively 1, wherein large common terminal lead and a plurality of relathe ratio of the sheet resistivity of said third layer to tively much narrower elongated terminal leads, that said first layer is between 2.7 and 10. each of said rings being connected to said common 7. A thermographic print head as described in claim terminal lead and to a separate one of said elon- 6, wherein gated terminal leadsthrough a selected pair of said said second layer has a thickness of about 50,000A, vias, respectively, the vias in each of said pairs of and vias being spaced 180 from each other, with re- 10 said first layer has a sheet resistivity of between 0.075 spect to the center of said rings. and 0.185 ohms per square. 5. A thermographic print head as described in claim 8. A thermographic print head as described in claim a 4, wherein 1, wherein each of a plurality of said elongated terminal leads said first layer is a composite layer ofa relatively thin extends in a radial direction from a point in comlower layer of chromium and a relatively thicker mon, upper layer of molybdenum. said elongated terminal leads have exposed terminal 8. A thermographic print head as described in claim connecting portions that are parallel to each other, 1, wherein and said first and said third layers comprise molybdenum. a fourth layer of electrically insulating material is dis- 

1. A thermographic print head comprising: an electrically insulating substrate, a first layer of electrically conductive material comprising a plurality of separated terminal leads disposed on a surface of said substrate, a second layer of electrically insulating material disposed over portions of said separated terminal leads and portions of said surface of said substrate between said terminal leads, a plurality of vias formed in said second layer, each of said vias extending to said first layer, and a third layer of electrically conductive material comprising a plurality of separated conductors disposed over said second layer, each of said conductors extending to, and contacting, two of said terminal leads through said vias.
 2. A thermographic print head as described in claim 1, wherein a fourth layer of electrically insulating material is disposed over said conductors.
 3. A thermographic print head as described in claim 2, wherein each of said second and fourth layers comprises sputtered glass having a melting point in excess of 490*C and a thermal expansion of about 3.3 X 10 6/*C, and said second layer is a composite of a bottom layer of said sputtered glass and a top layer of chemically vapor-deposited borosilicate glass.
 4. A thermographic print head as described in claim 1, wherein said plurality of conductors comprises a plurality of concentric rings, and said plurality of terminal leads comprises a relatively large common terminal lead and a plurality of relatively much narrower elongated terminal leads, each of said rings being connected to said common terminal lead and to a separate one of said elongated terminal leads through a selected pair of said vias, respectively, the vias in each of said pairs of vias being spaced 180* from each other, with respect to the center of said rings.
 5. A thermographic print head as described in claim 4, wherein each of a plurality of said elongated terminal leads extends in a radial direction from a point in common, said elongated terminal leads have exposed terminal connecting portions that are parallel to each other, and a fourth layer of electrically insulating material is disposed over said rings.
 6. A thermographic print head as described in claim 1, wherein the ratio of the sheet resistivity of said third layer to that said first layer is between 2.7 and
 10. 7. A thermographic print head as described in claim 6, wherein said second layer has a thickness of about 50,000A, and said first layer has a sheet resistivity of between 0.075 and 0.185 ohms per square.
 8. A thermographic print head as described in claim 1, wherein said first layer is a composite layer of a relatively thin lower layer of chromium and a relatively thicker upper layer of molybdenum.
 9. A thermographic print head as described in claim 1, wherein said first and said third layers comprise molybdenum. 